The present market for spherical ceramic particles is dominated by spherical silica because silica spheres are available in a wide variety of sizes at low cost. However, the thermal conductivity, electrical conductivity, and other properties of silica are not optimal for many important applications. Alumina (Al.sub.2 O.sub.3), for example, has a higher thermal conductivity and lower electrical conductivity than silica, and would likely replace spherical silica for some applications if a low-cost, commercial scale method of preparing spherical Al.sub.2 O.sub.3 of a controlled size of about 10-200 micron (.mu.m) diameter, and a controlled particle distribution were available.
Methods for preparing spherical Al.sub.2 O.sub.3 particles are known. These methods generally require plasmas generated from both high power (about 10 kW) and low power (about 1 kW) sources. Although spherical Al.sub.2 O.sub.3 particles can be generated by high power methods such as ablation from aluminum electrodes or vaporization of powdered precursors that nucleate and grow to form spheres require high power sources, control of spherical particle size is not possible using high power methods.
The use of low power microwave plasma to produce spherical A.sub.2 O.sub.3 was reported by H. Shim et al. entitled "Restructuring of Alumina particles Using a Plasma Torch", which appeared in J. Mat. Res. 14, 849 (1999), and in U.S. Pat. No. 5,989,648 to J. Phillips entitled "Plasma Generation of Supported Metal Catalysts," which issued on Nov. 23, 1999. The teachings of both Shim et al. and the '648 patent are incorporated by reference herein. Both Shim et al. and the '648 patent describe crystalline spherical Al.sub.2 O.sub.3 particles that were generated by exposing precursor Al.sub.2 O.sub.3 powder particles to a low power (&gt;1 kW) microwave plasma. A comparison of the particle size/particle size distributions of the powder particles used with the spherical product particles produced for both the Shim paper and the '648 patent shows that the average input powder particle is larger than the product spherical particle. Importantly, the data strongly suggest that each spherical particle was produced from a single precursor powder particle.
Clearly, a low cost method for generating micron scale particles of aluminum oxide, magnesium oxide, titanium oxide, iron oxide, nickel oxide, and other oxide ceramics of a controlled particle size and particle size distribution is highly desirable.
Therefore, an object of the present invention is a general, low cost method for generating spherical oxide ceramic particles in the micron size range.
Another object of the invention is a method of making oxide ceramic particles having a controlled size and size distribution.
Still another object of the invention is a method of making spherical alumina particles having a controlled size and size distribution in the micron size range.
Yet another object of the invention is a method of producing irregularly shaped, hard, ceramic agglomerates having macroscopic pores and of a controlled particle size and particle size distribution.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.